Simple Interest - General Questions

In how many years will a sum of money double itself at 12.5% per annum simple interest?

A sum of money amounts to Rs. 6,240 in 2 years and to Rs. 8,320 in 6 years at simple interest. What is the sum?

A sum of Rs. 18,000 earns a simple interest of Rs. 5,400 in 5 years. What is the rate of interest per annum?

A sum of Rs. 3,000 becomes 3 times itself in 8 years at simple interest. What is the rate of interest?

A sum of Rs. 7,000 becomes 4 times itself in 5 years at simple interest. What is the rate of interest?